1. Field of the Invention
The present invention relates to a laminated electric element and, more particularly, to a laminated electric element which contains a plastic film provided on one surface or both surfaces thereof with a conductive layer, and a foam layer laminated on the plastic film through a middle layer.
2. Brief Description of the Prior Art
A variety of electric elements utilizing electric properties of plastic films have been developed and commercialized. The term "electric properties" referred to herein and throughout the specification unless otherwise stated may include not only electric properties inherent in plastic films, such as dielectric constant and electric resistance, but other electric properties acquired a posteriori, such as electret properties (performance with respect to charges on the surface of plastic film) produced therein by treating the plastic film, for example, under high direct current electric field, and piezoelectric and pyroelectric properties.
Electric elements are used usually by bringing the plastic films into contact with or taking them away from electrodes for the input and the output of electric signals or a common electrode for capacity coupling or a covering for such electrodes. The plastic film may often be employed in the form of a metallized plastic film which comprises a base plastic film and a thin metal film or conductive layer deposited on one surface or on both surfaces thereof as an electrode layer.
One of the purposes for using the metallized film is to utilize properties that the film is so soft or flexible that it is likely to be varied in form with a stress. In order to produce a stress readily in the film by applying a pressure thereto or to provide the film with a cushioning effect, a material, such as a plastic foam, which can act as a cushioning material and has an inwardly and outwardly curved or waved surface, may have been employed by depositing an electrode layer or conductive layer thereon.
Where the plastic film in the electric element has a thickness thicker than a certain level which is to be determined in accordance with a Young's modulus, on the one hand, the surface of the plastic film can be held flat without being subject to deformation resulting from the curved surface of a foam layer laminated thereon. If the plastic film has a thickness thinner than such certain level, on the other hand, the surface of the flexible or soft plastic film may become curved or waved along with the curved surface of the foam layer, whereby the electrode layer or conductive layer deposited on the plastic film also becomes curved or waved.
The curved or waved plastic film and/or electrode layer may adversely affect performance as a laminated electric element. Where it is employed with an infrared sensor, an infrared image to be formed on the metal film or conductive layer may tend to become blurred or faded. With an acoustic transducer which is provided with such waved film and/or layer, sounds may tend to become warped or deflected. A capacitive switch employing such curved or waved plastic film and/or electrode layer may tend to provide no desired electric capacity. In each case, there may be a tendency that the electrode layer tends to come off because it is attached to the plastic layer with a relatively weak adhesive force.